vented box & loudspeaker stuffing

[sstoiana]

*newbie*
I've built a few boxes, mostly by ear, with some degree of succes. However, I've had a few problems with vented designs (cone excursion exceeding xmax
I have just discovered WinISD, and was experimenting with some vented box designs. Thing is, I don't really understand how Qa works.
The help file states :
"Qa, absorption losses. These are produced by losses in enclosure. Any stuffing will increase the absorption loss. With no stuffing inside the box, 100 is typical. Heavily stuffed enclosure has this about 3-5. "
What does "heavily stuffed enclosure" mean ? That there is only stuffing inside the box ? Then what would Qa=50 mean ? 50% of the box is filled with stuffing material ?

Another question:
What material is best for stuffing ? I've seen (and heard of) at least a dozen different materials used, from virgin wool to polyurethane foam, fluffed cotton fibers to chopped up rags. What's best ? And how would you fix it inside the box ?

Any help is apreciated!

[SisterOfMercy]

I use pritex, which I think is some sort of polyurethane foam.. I fix it to the cabinet with bitumen plates that have double sided adhesive..